Dynamic isolation monitoring with low sensitivity to noise

1. A monitoring circuit for a battery system, the circuit comprising:

a voltage divider circuit that is configurable;

a voltage measurement circuit;

a sampling circuit; and

a microcontroller, wherein the microcontroller is configured to:

connect the voltage divider circuit to a battery of the battery system;

initiate a measurement of a first battery voltage by the voltage measurement circuit;

calculate a settled value of a first chassis voltage using sampled values of the first chassis voltage sampled using the sampling circuit;

change a configuration of the voltage divider circuit;

initiate a measurement of a second battery voltage;

calculate a settled value of a second chassis voltage using sampled values of the second chassis voltage; and

calculate isolation impedance of the battery using the first battery voltage, the second battery voltage, and the calculated settled values of the first chassis voltage and the second chassis voltage.

2. The circuit of claim 1 , wherein the microcontroller is further configured to:

calculate a derivative of the sampled values of the first chassis voltage and calculate a derivative of the sampled values of the second chassis voltage;

determine a first ratio of consecutive derivatives for the first chassis voltage and determine a second ratio of consecutive derivatives for the second chassis voltage;

determine a first decay value using the first ratio and determine a second decay value using the second ratio; and

calculate the first chassis voltage using the first decay value and calculate the second chassis voltage using the second decay value.

3. The circuit of claim 1 , wherein the microcontroller is further configured to:

calculate a first difference of the sampled values of the first chassis voltage and calculate a second difference of the second chassis voltage;

determine a first ratio of consecutive first differences and determine a second ratio of consecutive second differences;

determine a first decay value using the first ratio and determine a second decay value using the second ratio; and

calculate the first chassis voltage using the first decay value and calculate the second chassis voltage using the second decay value.

4. The circuit of claim 3 , wherein the microcontroller is further configured to:

calculate a first filtered ratio of the consecutive first differences;

calculate a second filtered ratio of the consecutive second differences; and

determine the decay value using the filtered ratio.

5. The circuit of claim 1 , wherein the microcontroller is further configured to:

filter the sampled values of the first chassis voltage;

filter the sampled values of the second chassis voltage; and

calculate the settled values of the first chassis voltage and the second chassis voltage using filtered sampled values of the first chassis voltage and the second chassis voltage.

6. The circuit of claim 1 , wherein the microcontroller is further configured to:

compare the calculated isolation impedance to a threshold; and

generate an alert when the determined isolation impedance is less than the threshold.

7. The circuit of claim 1 , wherein the battery system is a battery system for an electric vehicle, and the resistive divider circuit is configured to electrically connect the battery of the battery system to a chassis of the electric vehicle through the voltage divider circuit.

8. The circuit of claim 1 , wherein the sampling circuit includes a sample and hold circuit and an analog-to-digital converter circuit.

9. The circuit of claim 1 , wherein the voltage divider circuit includes:

multiple series connected resistors including a first resistor and a second resistor connected by a circuit node, and the multiple series connected resistors are connected in parallel to the battery; and

a switching circuit coupled to the circuit node;

wherein the microcontroller is configured to change the configuration of the resistive divider circuit by activating the switching circuit to connect the circuit node to circuit ground.

10. The circuit of claim 1 , wherein the voltage divider circuit includes:

a first resistor and a first switch coupled in series to a first battery terminal of the battery;

a second resistor and a second switch coupled in series to a second battery terminal of the battery;

a third resistor coupled to the first and second switch circuits and to circuit ground; and

wherein the microcontroller is configured to change the configuration of the resistive divider circuit by changing from activating the first switch and deactivating the second switch to deactivating the first switch and activating the second switch.

11. A method of monitoring a battery system, the method comprising:

connecting a voltage divider circuit to a battery of the battery system;

measuring a first battery voltage;

sampling a first chassis voltage for less than a settling time of the first chassis voltage and estimating a settled value of the first chassis voltage using sampled values of the first chassis voltage;

changing a configuration of the voltage divider circuit;

measuring a second battery voltage;

sampling a second chassis voltage for less than a settling time of the second chassis voltage and estimating a settled value of the second chassis voltage using sampled values of the second chassis voltage; and

determining isolation impedance of the battery to a chassis using the first and second battery voltages and the estimated settled values of the first and second chassis voltages.

12. The method of claim 11 , wherein estimating the settled value of either the first chassis voltage or the second chassis voltage includes:

calculating a derivative of the sampled values of the chassis voltage;

determining a ratio of consecutive derivatives of the sampled values; and

determining a decay value using the ratio and calculating the settled value of the chassis voltage using the determined decay value.

13. The method of claim 11 , wherein estimating the settled value of either the first chassis voltage or the second chassis voltage includes:

calculating a difference of sampled values of the chassis voltage and determining a ratio of consecutive differences of the sampled values; and

determining a decay value using the ratio and calculating the settled value of the chassis voltage using the determined decay value.

14. The method of claim 13 , including filtering the ratio of the consecutive differences of the sampled values, and determining the decay value using the filtered ratio.

15. The method of claim 11 , including:

filtering the sampled values of the first chassis voltage and the sampled values of the second chassis voltage; and

estimating the settled values of the first chassis voltage and the second chassis voltage using filtered sampled values of the first chassis voltage and the second chassis voltage.

16. The method of claim 11 , including:

comparing the determined isolation impedance to a threshold; and

generating an alert when the determined isolation impedance is less than the threshold.

17. The method of claim 11 , wherein the battery system is a battery system for an electric vehicle, and wherein the method further includes electrically isolating the battery from the chassis in an operating mode and electrically connecting the battery to the chassis through the voltage divider circuit in a test mode.

18. A monitoring circuit for a battery system, the circuit comprising:

a voltage measurement circuit;

a sampling circuit;

a resistive divider circuit;

a microcontroller circuit, wherein the microcontroller is configured to:

initiate a measurement of a first battery voltage by the voltage measurement circuit;

calculate a settled value of a first chassis voltage using sampled values of the first chassis voltage sampled using the sampling circuit;

change a configuration of the resistive divider circuit;

calculate a settled value of a second chassis voltage using sampled values of the second chassis voltage; and

calculate isolation impedance of the battery using the battery voltage, the calculated settled value of the first chassis voltage, and the calculated settled value of the second chassis voltage.

19. The circuit of claim 18 , wherein the voltage divider circuit includes:

a first resistor and a first switching circuit coupled in series to a first battery terminal of the battery;

a second resistor and a second switching circuit coupled in series to a second battery terminal of the battery;

a third resistor coupled to the first and second switching circuits and to circuit ground; and

wherein the microcontroller is configured to change the configuration of the resistive divider circuit by changing from activating the first switching circuit and deactivating the second switching circuit to deactivating the first switching circuit and activating the second switching circuit.

20. The circuit of claim 18 , wherein the microcontroller circuit is further configured to:

initiate a measurement of a second battery voltage after the change in configuration of the resistive divider circuit; and

calculate isolation impedance of the battery using the first battery voltage, the calculated settled value of the first chassis voltage, the calculated settled value of the second chassis voltage, and the second battery voltage.

21. The circuit of claim 18 , wherein the resistive divider circuit includes:

multiple series connected resistors including a first resistor and a second resistor connected by a circuit node, and the multiple series connected resistors are connected in parallel to the battery; and

a switching circuit coupled to the circuit node; and

wherein the microcontroller is configured to change the configuration of the resistive divider circuit by activating the switching circuit to connect the circuit node to circuit ground.

22. The circuit of claim 18 , wherein the microcontroller is configured to:

calculate a difference of sampled values of at least one of the first or second chassis voltages and determine a ratio of consecutive differences of the sampled values of the at least one of the first or second chassis voltages; and

determine a decay value using the determined ratio and calculate the settled value of the at least one of the first or second chassis voltages using the determined decay value.